1. Technical Field of the Invention
The present invention relates generally to the field of aeronautics, and, more particularly, to passive flow control of a medium about a bluff body disposed on a generally planar surface.
2. Description of Related Art
Even before World War II flow control devices were being created to help control turbulence created by the flow of air over an airfoil such as a wing of an aircraft. An aircraft attains lift by forcing air over a shaped wing at a higher rate of speed than the air passing below the wing. Since the air above the wing travels at a higher rate than the air below, an area of high pressure is developed below the wing and an area of low pressure is created above the wing resulting in an upwardly directed lifting force. As the two air flows merge back together aft of the wing, turbulence and/or unstable vortices form at higher incidences to the direction of travel which can extend for long distances behind the moving wing. Other devices of varying spherical shapes mounted to or protruding from the wing or fuselage cause similar turbulence as the airflow moves across and around the protruding devices and separates to form a turbulent wake. This turbulence reduces the aerodynamic performance, by reducing lift and control as well as increasing drag of an aircraft.
The structure of a turbulent boundary layer has been studied by numerous investigators and its description in terms of rational structures is subject to much debate. Though turbulence is a very complex phenomenon, a specific set of features in the turbulent boundary layer has been identified: streamwise vortices, wall layer streaks, ejections, breakups, sweeps, outer layer motions and their interactions with the wall and entrainment of irrotational flows. The creation of turbulence has usually been referred to as the bursting phenomenon. One approach to interpreting turbulence control experiments is to view them as attempts to interfere with some component of the turbulence creation cycle. Thus, one can think of modifying either the large outer vortices or the wall layer elements, thus, changing the communication between the various scales.
Many advantages are realized by improving the aerodynamic performance of an aircraft. For example, a reduction in drag will reduce the fuel necessary to power the aircraft. Since, millions of dollars are spent each year on fuel to power commercial and military aircraft, thus, even small decreases in fuel consumption results in large fuel cost savings. Current drag control devices which have been developed include laminar flow control using suction, blowing, polymer additive compliant walls, textured surfaces, and riblet structures. Practical application of these devices generally spawn complex parts, high cost, added weight, maintainability problems and have not been extended to control turbulence about a round or spherical type body positioned on an aerodynamic surface.
The above-mentioned and other disadvantages of the prior art are overcome by the present invention, by providing a fully passive device to control turbulence requiring only low profile wall modifications.
The present invention achieves technical advantages as an apparatus and system for controlling turbulence generated by medium passing about a bluff body disposed on a generally planar flow surface. The system uses a forward curved vortex fence and an aft fairing all of which have a predefined width of less than that of the bluff body. The maximum height of the forward curved vortex fence and aft fairing is a fraction of the bluff body. The forward curved vortex fence is disposed on the flow surface in front of the bluff body extending forward. The curved vortex fence is positioned most near the forward face of the bluff body, but, allowing a predetermined gap between the two. The gap is a fraction of the bluff body height. The aft fairing is disposed on the flow surface in back of the bluff body with the leading edge positioned most near the aft face of the bluff body with a minimum practical gap or no gap at all. The length of the aft fairing has a leading edge and trailing edge in which the height tapers continuously from a maximum at the leading edge to a minimum at the trailing edge.
In another embodiment of the present invention, the system and method for controlling turbulence generated by medium passing about a bluff body disposed on a generally planar flow surface includes a forward fairing integrated with the forward curved vortex fence and the above-described aft fairing. The forward fairing also has a predefined width and height of less than that of the bluff body. The length of the forward fairing has a leading edge and a trailing edge in which the height tapers continuously from a minimum at the leading edge to a maximum equal to the curved vortex fence height at the trailing edge. The forward fairing is disposed on the flow surface in front of the bluff body with the trailing edge integrated with the forward curved vortex fence and positioned most near the bluff body, but, allowing a predetermined gap between the two.
In practice, the flow surface is propelled in a forward direction to develop medium flow from the front side of the bluff body around the outboard sides of the bluff body and into a wake behind the bluff body. Without the curved vortex fence and aft fairing with or without the and/or forward fairing, the wake is normally very turbulent and is a low-energy flow, both of which are caused by the blockage effect of the bluff body. The curved vortex fence and aft fairing with or without the forward fairing work together to modify and organize occurring flow features in such a manner as to provide a pumping effect that redirects flow of high energy non-turbulent medium from around the outboard sides of the bluff body into the low energy turbulent wake behind the bluff body.
The occurring flow features that are modified and organized by the present invention include, but are not limited to, (1) the boundary layer flow that exists on the flow surface upstream of the bluff body, and (2) an unstable xe2x80x9chorse-shoexe2x80x9d vortex that exists along the corner of the intersection of the bluff body and the flow surface. The modified and organized flow features provide reduced drag, reduced energy required to propel the flow surface and/or improved control of the flow surface.
Among the new advantages of the present invention are: no moving components are required, reducing the cost and complexity of non-passive devices; only low profile wall modifications are required to practice the invention, thus, parasitic drag is reduced or eliminated; and drag is reduced, consequently, energy required to propel the flow surface is reduced and control of the flow surface is improved.